Is it possible to assign an active camera through command line rendering?
You can set your camera in a textblock in your blend file text editor using 

import bpy
bpy.context.scene.camera = bpy.data.objects["Camera"]


then call run your .blend file from the command line and run this textblock as shown below:

blender --background myfile.blend --python-text Text

where myfile.blend is your blend file and Text
 is your textblock name and --background is an argument to run your .blend file in the background without opening blender GUI

If you want to set a specific camera without having to go back and forth then you can pass the camera name you want in your command line as shown here